1. Field of the Invention
The present invention relates to a multi-nozzle ink jet recording device, wherein ink droplets are charged by a charger electric field at the time of ejection and deflected by a deflector electric field so as to control impact positions of the ink droplets, thereby providing a high quality image.
2. Description of the Related Art
As disclosed in Japanese Patent Publication No. SHO-47-7847, there has been proposed a conventional ink jet recording device wherein ink droplets, which are uniform in size and separated from one another, are ejected through nozzles in response to a print signal, charged by a charger electric field in accordance with the print signal, and deflected by a constant deflector electric field so as to either collect the ink droplets before impacting on a recording medium or control impact positions of the ink droplets on the recording medium. In order to improve the printing speed, a plurality of nozzles are arrayed.
In a serial printing type ink jet recording device, the process of the head to print while scanning across the recording medium and the feeding process to feed the recording sheet are repeatedly performed in alternation so as to from a complete image.
When there is uneven characteristic among the nozzles, ejected direction of ink droplets varies among the nozzles. This varies the impact positions of the ink droplets on the recording medium and results in uneven ink density on the image. Undesirable strips extending in the head scanning direction appear and image quality is degraded. In order to overcome this problem, a multipath printing method is used. That is, a print region that is printed in a single scan is overlapped with neighboring print regions, and dots on or near the same scanning line are formed by a plurality of nozzles in alternation during the scan and the subsequent scan. In this way, the variations in characteristics of the different nozzles will be cancelled out, and so the uneven ink density in the printed image is suppressed.
Arraying the nozzles is effective in improving printing speed. When the print head is elongated to have a width corresponding to the width of the recording medium, there is no need to scan the head across the recording sheet at all, and printing is performed while feeding the recording medium continuously. This type of printing is called line printing, and is excelling in printing speed. However, there are a number of problems to overcome before realizing the line printing type ink jet recording device.
One of the problems is the fact that the multipath printing method cannot be used in the line printing type ink jet recording device, because dots on a single scanning line in the sheet feed direction are formed only by a corresponding one of the nozzles. Therefore, if an impact position of ink droplets from any nozzle shifts from a target position, a distinct strip extending in the sheet feed direction appears in printed images. It is conceivable to align a plurality of heads in parallel in order to obtain the same effect as the multipath printing. However, this makes the recording device undesirably bulky and is not realistic way to solve the problem.
Japanese Patent-Application Publication Nos. SHO-55-42836, HEI-2-62243, and HEI-7-117241 proposes methods of solving the above problem, wherein a pseudo borderline is defined between the print regions allocated to the neighboring nozzles, which differs from an actual borderline. The pseudo borderline is in a saw shape, which has a certain amplitude and a repetition frequency. Because the adjacent print regions protrude and retract, the unevenness in ink density can be less recognizable.
However, usually the resolution at the border degrades in the conventional recording device. Some images, the alaising of the image itself interferes with the pseudo borderline in the saw shape, resulting in degradation in image quality. This problem is especially remarkable when high-resolution imagers or dot half-tone images are printed.
Moreover, no matter what type of saw-shaped border is used, when impact positions are undesirably separated from adjacent impact positions, then a line extending along the saw-shaped border appears. Although the saw-shaped line is less likely noticed compared with the straight line, the saw-shaped line appeared in all black images will be distinct.
It is an object of the present invention to overcome the above problems, and also to provide a line printing type ink jet printer capable of forming high quality images without uneven ink density causing white or black density.
In order to achieve the above and other objectives, there is provided an ink jet recording device including a head, an electric field generating means, an instructing means, and a signal processing means. The head is formed with a plurality of nozzles aligned in a first direction, and selectively ejects ink droplets from the nozzles in response to an ejection data to form an image on a recording medium. The electric field generating means generates a charger electric field for charging the ink droplets and a charger electric field for deflecting a flying direction of the charged ink droplets in response to a deflection data. The electric field generating means includes an electrode provided common to the plurality of nozzles and extending in the first direction. The instructing means outputs an instruction indicating an overlapping manner of a plurality of dots of ink droplets ejected from different nozzles to form a single dot. The signal processing means generates the ejection data and the deflection data based on the instruction from the instructing means.
There is also provided an ink jet recording device including a head, deflecting means, a moving unit, an instructing means, and a signal processing means. The head is formed with a plurality of nozzles aligned in a first direction, and selectively ejects ink droplets from the nozzles onto a recording medium in response to ejection data. The deflecting means deflects a flying direction of the ejected ink droplets toward a second direction perpendicular to the first direction in response to deflection data. The moving unit relatively moves the recording medium in a third direction angled from the first direction. The instructing means instructs an overlapping manner of dots of a plurality of ink droplets for forming a single dot. The signal processing means generates the ejection data and the deflection data based on the instruction from the instructing means.